1. Field of the Invention
The present invention relates to a surface emitting laser device and a method of fabricating the surface emitting laser device, and more particularly, to a long wavelength vertical cavity surface emitting laser device for optical communication, which has a band in the range of 1.3 to 1.6 μm, and a method of fabricating the long wavelength vertical cavity surface emitting laser device.
This work was supported by the IT R&D program of MIC/IITA. [2005-S-051-02, Photonic device integrated module for optical access network]
2. Description of the Related Art
Surface emitting laser devices emit a beam in a direction perpendicular from a top surface of a substrate, unlike edge emitting laser devices emitting asymmetrically shaped beams and having weak coupling efficiency with respect to optical fibers. Vertical-cavity surface emitting laser (VCSEL) devices, which generate output light by a vertical cavity, have a high coupling efficiency due to a circular beam shape and a low threshold current, as compared with conventional edge emitting laser devices. Since a small current of several milliamps (mA) can drive VCSEL devices, a general CMOS IC can be used to directly drive the VCSEL devices, thereby reducing the costs. The VCSEL devices can be easily fabricated as two-dimensional array devices, and can be mass-produced and tested on a wafer-by-wafer basis, unlike in the case of edge emitting laser devices. Thus, since VCSEL devices can be sufficiently mass-produced, they have been developed as a replacement of conventional laser diodes (LDs).
However, since VCSEL devices for emitting light having wavelengths in the range of 850 to 980 nm have limits with respect to a distance and velocity at which light moves through an optical fiber, these VCSEL devices are used only for local area communication, and are difficult to be used in long-distance optical communication. Accordingly, VCSEL devices for emitting light having long wavelengths in the range of 1.3 to 1.6 μm, which have a communication distance of several kilometers, have been developed as a light source that overcomes the limits of conventional VCSEL devices having a band of around 850 nm. For example, in order to ensure continuous wave operation in a temperature range of a room temperature to a high temperature of 85␣, and also to ensure a rapid operational velocity and a high output power, research has been conducted into various ways to achieve this, such as a method for reducing a threshold current and a series resistance, an effective heat release method, an appropriate optical inducing method and the like.
Long wavelength VCSEL devices for emitting light with a wavelength in the range of 1.3 to 1.6 μm are classified into devices including InP substrates and GaAs substrates. Thus, a mirror layer having a high reflectivity and a gain medium having a high optical gain, which are required by the long wavelength VCSEL devices, vary according to whether the InP substrate or the GaAs substrate is used.
In a device embodied on a GaAs substrate, a reflective mirror formed of a pair of GaAs/Al(Ga)As layers can be grown, wherein there is a large difference in the refractive indexes of the pair, an optical/carrier confinement structure having good performance can be embodied by using a layer generated by wet-oxidizing AlAs, and an active layer providing an output light gain has a large band-gap energy. Thus, the performance of the device does not deteriorate due to heating. An example of a GaAs-based device is a device in which an InGaAsN/GaAsN quantum well structure is used as a gain medium.
However, as nitrogen increases or the wavelength of oscillating light increases, the gain and stability of the gain medium can deteriorate, and it is difficult to obtain gain properties of a long wavelength in the range of 1.3 μm or more.
At present, surface emitting laser devices are mainly fabricated using an InGaAsP or InAlGaAs material grown on InP substrate. In the case of surface emitting layer devices including an InP substrate, it is difficult to obtain a pair of materials constituting a reflective mirror, which have a large difference between their refractive indexes. Thus, many layers needs to be grown so as to obtain a high reflectivity. For example, pairs of InAlGaAs/InAlAs layers and InAlGaAs/InP layers have been used.
In addition, since quaternary materials such as InGaAsP and InAlGaAs, which are used as materials for active layers or reflective mirrors, have low thermal conductivity corresponding to 0.1 times that of a binary material such as GaAs, the heat releasing properties of a device may deteriorate. In long wavelength VCSEL devices, a unique small band gap of a material results in a remarkable decrease in the performance of a device and an increase in the temperature. Thus, effective heat release is particularly important in long wavelength VCSEL devices.
Meanwhile, a method of manufacturing a surface emitting laser device is largely classified into a monolithic method and a hybrid method. An example of the hybrid method is a method in which an active layer providing optical gain and a mirror layer are separately grown and are then bonded to each other. In this case, by growing separate structures using separate processes (for example, a quaternary material is used as a long wavelength gain material, and a binary material such as GaAs/AlAs having a large difference in refractive indexes is used to form a mirror layer), the advantages of a GaAs substrate type laser device and an InP substrate type laser device can be mixed, and good thermal and optical properties can be obtained.
However, wafer-bonding needs to be performed in order to bond two wafers that have been epitaxially grown in different processes. Therefore, erroneous wafer bonding may occur, the reliability and mass-productivity may be reduced, and costs may increase. In other words, when wafers formed of hetero semiconductor such as GaAs and InAlGaAs are bonded or metamorphically grown, since a bonded region of the two wafers plays an electrically and optically sensitive role in a laser emitting structures reliability and mass-productivity may be reduced, and as a result, the cost of chips may increase.
Meanwhile, the monolithic method is a method in which structures such as a mirror layer and an active layer are sequentially grown on same semiconductor substrate using a semiconductor epitaxial growth method. Fabrication processes can be simplified by using the monolithic method, but heat release properties can deteriorate due to the fact that it is difficult to grow a thick mirror layer and quaternary materials are used.